People who get too close to my shopping cart at the grocery store

My pitbull Muffins, service animal, sits in my shopping cart while I’m at the grocery store. She is very protective of my groceries and growls at other shoppers when they get too close to my cart.

Sometimes they complain to the manager but why are they getting so close to my cart? Muffins doesn’t know what their intentions are, possibly stealing items out of the cart?